Generally, a window is a device located in an opening in a wall or door that allows the passage of light. Windows are usually glazed or covered in some other transparent or translucent material like a float glass. Windows are held in place by frames, which prevent them from collapsing in. Many glazed windows may be opened, to allow ventilation, or closed, to exclude inclement weather.
Many windows include glazings and/or fillings. Common types of glazing that are used in architectural applications include clear and tinted float glass, tempered glass, and laminated glass as well as a variety of coated glasses, all of which can be glazed singly or as double, or even triple, glazing units. Ordinary clear glass has a slight green tinge but special clear glasses are offered by several manufacturers.
Since windows allow light to pass therethrough, a significant amount of heat may be accumulated inside of a structure having the window, especially when the window is oriented to receive incident sunlight. As such, various glazings and coatings have been designed to decrease the amount of sunlight that is transmitted through the window. Low-emissivity coatings can reduce heat transfer by sunlight radiation, which, depending on which surface is coated helps prevent heat loss (in cold climates) or heat gain (in warm climates). High thermal resistance can be obtained by evacuating or filling insulated glazing units with gases such as argon or krypton, which reduces conductive heat transfer due to their low thermal conductivity. The thermal performance of windows that have glazings depends on good window seals and meticulous frame construction to prevent entry of hot or cold air and loss of efficiency. Modern double-pane and triple-pane windows often include one or more low-e coatings to reduce the window's U-factor. In general, soft-coat low-emissivity coatings tend to result in a lower solar heat gain coefficient (SHGC) than hard-coat low-coatings.
Additionally, the angle at which light from the sun strikes a window can be determinative of the amount of heat that can accumulate within a structure. Historically, windows are usually designed with surfaces parallel to vertical building walls. Such a design allows considerable sunlight and heat penetration due to the most commonly occurring incidence of sun angles when the window is oriented so as to receive sunlight. In passive solar building design, an extended eave over a window is typically used to control the amount of sunlight and heat entering the window(s). An alternate method would be to calculate a more optimum angle for mounting windows which accounts for summer sunlight load minimization, with consideration of the actual latitude of the particular building. An example where this process has been implemented is the Dakin Building, Brisbane, Calif.; much of the fenestration has been designed to reflect summer sunlight heat load and assist in preventing summer interior over-illumination and glare, by designing window canting to achieve a near 45 degree angle with respect to the ground.
While there have been various advances in the art of windows and glazings to inhibit sunlight from passing therethrough, none of the advances have found a solution that also allows significant sunlight transmittance during colder winter months.